Centrifugal pumps may be used in a number of applications involving the pumping of suspensions having a fiber content. In the paper making industry, for example, centrifugal pumps have been used for many years to pump the fibrous pulp slurries that are used in the paper making process from one processing station to another. In different configurations, centrifugal pumps have been utilized to pump slurries or suspensions having varying consistencies of wood pulp content, ranging from less than 1% to nearly 20% by weight.
At certain points in the paper making process, or at all points in certain processes, it can be advantageous to maintain the suspension at a high consistency. In such situations, it may be economical and beneficial to employ a centrifugal pump, as opposed to a positive displacement pump or other type of pump, to pump the fiber suspension. Centrifugal pumps of conventional design, i.e., designed for pumping clear liquid, can successfully pump from a vessel fiber suspensions having consistencies as high as 3% to 10% with at most only minor modifications to certain dimensions and clearances depending upon the suction pressure, the type of fiber, the air content and other factors. Aside from those factors, however, such a conventional pump is not able to adequately move fiber suspensions above a certain consistency from the vessel and through the pump's inlet pipe to the pump's impeller. This is due to the fiber suspension's tendency to create flocs or clumps of fibers. The fibers and fiber flocs inherently entangle to form networks or structures that do not flow as a Newtonian liquid. In particular, the flocs have difficulty passing from the vessel interior into the pump's inlet pipe or nozzle which leads to the eye of the impeller. In effect, a bottle-neck occurs at the transition from the vessel to the pump inlet pipe at which point the fiber network tends to clog and grow in size until movement of fiber suspension into the pump inlet is reduced or stopped. This reduces the pump output or can cause the pump to lose its prime and cease pumping altogether.
Most prior efforts to overcome the above-mentioned problem of centrifugally pumping high consistency fiber suspensions have focused on providing a rotating device upstream of the pump impeller and near the transition from the vessel outlet to the pump's inlet pipe or nozzle. Such rotating devices, which have been referred to as propellers, feeders, fluidizers, breaker impellers and the like, serve to create a shear rate in the suspension sufficient to cause additional relative movement between flocs near the transition from the vessel outlet to the pump's inlet pipe. This additional relative movement or agitation of flocs caused by the rotating device prevents clogging at the transition with fiber suspensions having consistencies considerably higher than would be possible without the rotating device. For example, the provision of such a rotating device can enable a centrifugal pump to accommodate suspensions with ccnsistencies as high as nearly 20%.
Examples of prior centrifugal pumps having rotating devices as described above are shown in U.S. Pat. Nos. 4,780,053 and 4,854,819. In some cases, the rotating device has been configured with a twist or pitch to provide, in addition to the agitation described above, a force on the suspension through the inlet pipe and toward the eye of the impeller. Certain prior configurations included a plurality of such rotating devices, while others utilized a single rotating device. In some prior configurations, the rotating device was driven by an extension of the shaft driving the pump impeller. In others, a separate drive shaft was provided.
In any case, the prior configurations described above were the result of efforts to modify the conventional design to overcome the problems of clogging at high consistencies. As a result, although these pumps are capable of pumping the high consistency fiber suspensions, they have certain drawbacks and deficiencies.
One shortcoming of the prior pumps is that they are relatively complicated and have rotors which extend into the pump inlet pipe and, in some cases, into the vessel itself. As a result, the prior pumps included a significant amount of machinery simply for the purpose of agitating the suspension and, in some cases, urging it into the pump inlet pipe or toward the pump's impeller vanes. This has resulted in increased costs of manufacture and added complexity of manufacture, installation and servicing.
Another shortcoming of prior pumps is that the rotating device, by creating a shear rate in the suspension, requires a significant amount of power that does not contribute to the flow rate or pressure generated by the pump. As a result, a larger, more costly, motor may be required to drive the pump, or an additional motor may be required to drive the rotating device. Thus, the energy efficiency of the pump is adversely affected.
Yet another shortcoming of the prior pumps is that, by simply including a rotating device upstream of the impeller in a pump having an inlet pipe, the friction losses caused by the inlet pipe itself are not avoided. Even the minimal length of the inlet pipe of a standard centrifugal pump creates enough losses to severely impair or halt the performance of such pumps at high consistencies.